1. Field of the Invention
The present invention relates to an infrared radiation source wherein hot-pressed molybdenum disilicide reinforced with silicon carbide whiskers is used as the illuminant thereof, or a heating source wherein hot-pressed molybdenum disilicide reinforced with silicon carbide whiskers is used as the heating element thereof, and particularly to an infrared radiation source or heating source, which is prevented from low-temperature oxidation of terminal portions of the illuminant or heating element during use of the infrared radiation source or heating source to prolong the life span of the infrared radiation source or heating source, and which is applied to an infrared analyzer, a heater in an industrial furnace, or the like.
2. Description of the Related Art
Utilization of a molybdenum disilicide illuminant in a radiation source in a gas analyzer using infrared radiation has been attempted by making much of an advantage that a high luminance can be secured by heating it up to a high temperature. Since molybdenum disilicide has a low resistivity of 0.0003 .OMEGA..multidot.cm, however, a large electric current is necessary for heating molybdenum disilicide up to a high temperature, thus resulting in a large power consumption. Molybdenum disilicide also involves a practical problem that the shape of an illuminant or heating element made thereof cannot be retained because of its creep deformation at high temperatures.
A heating element capable of reducing the power consumption thereof to a low level, which is produced by forming molybdenum disilicide into a fine wire to thereby increase the apparent resistance thereof, has been proposed according to a technology of solving one of the foregoing problems in Japanese Patent Application Laid-Open No. 296,833/1993.
On the other hand, a technology of molybdenum disilicide composite reinforced with silicon carbide whiskers is disclosed as a method of suppressing the creep deformation of molybdenum disilicide at high temperatures in Japanese Patent Application Laid-Open No. 198,680/1996.
Meanwhile, a DC power source is usually used as a power source for supplying electricity to an illuminant in an infrared gas analyzer because it is important to generate a stable amount of infrared radiation. The inventors of the present invention have found out that, in many cases where an illuminant made of molybdenum disilicide reinforced with silicon carbide whiskers is heated up to and kept at a temperature necessary for infrared analysis, e.g., 1,300.degree. C., by supplying DC electricity to the illuminant, oxidation of a terminal portion thereof connected with the positive electrode of a direct current source and heated in the temperature range of 400 to 800.degree. C. not directly involved in infrared radiation emission, particularly at around 500.degree. C., preferentially proceeds to lose the function of flowing electricity in this portion, whereby the serviceable life span of a radiation source comprising the illuminant is completed.
An infrared radiation source in particular is desired to have a life span of at least 10,000 hours.
In view of this, any radiation source made of molybdenum disilicide capable of providing a high luminance while satisfying such a long life span, if obtained, is believed to serve to improve the precision of an infrared gas analyzer and hence to greatly contribute to analytical chemistry.
Molybdenum disilicide also is generally used as a heating element for an industrial furnace wherein a ceramic or the like is fired in the air. In this case as well, the heating element is often fractured because low-temperature oxidation of molybdenum disilicide, which is peculiar to molybdenum disilicide, proceeds in the low temperature range of at most 1,000.degree. C., particularly in a temperature range of around 500.degree. C. In order to prevent such low-temperature oxidation, the heating element made of molybdenum disilicide is usually preliminarily subjected to a pre-oxidation treatment at a high temperature of at least 1,000.degree. C. for formation of a dense silica film on the surface thereof, after which it is used. However, the low-temperature oxidation mentioned above occurs predominantly at a terminal portion of the illuminant on the positive electrode's side, although the protective silica film is formed on the surface by means of a pre-oxidation technique which is prevalently done for the heating element of an industrial furnace. This low-temperature oxidation leads to a fracture of the protective silica film and further proceeds inward.
Accordingly, an object of the present invention is to provide an infrared radiation source comprising an illuminant made of molybdenum disilicide reinforced with silicon carbide whiskers and having a long life span as well as a heating source comprising a heating element made of molybdenum disilicide reinforced with silicon carbide whiskers and having a long life span by suppressing the low-temperature oxidation phenomenon thereof which proceeds in a terminal portion thereof where it is connected with the positive terminal of a DC power source.
As described hereinabove, molybdenum disilicide can be used as a heat-resistant material in an atmosphere of air up to a high temperature of 1,800.degree. C. since the protective silica film can exhibit an excellent oxidation resistance in the atmospheric environment. When it is used in the low-temperature range of less than 1,000.degree. C., particularly in a temperature range of around 500.degree. C., low-temperature oxidation peculiar to molybdenum disilicide proceeds to fracture molybdenum disilicide. In view of the above, a method wherein molybdenum disilicide is preliminarily subjected to a pre-oxidation treatment at a high temperature of at least 1,000.degree. C. to form a dense silica film on the surface thereof has hitherto been employed with a view to preventing low-temperature oxidation of molybdenum disilicide. However, a long life span of molybdenum disilicide cannot be secured only by this method.
Specifically, when the infrared radiation source comprising the illuminant made of molybdenum disilicide reinforced with silicon carbide whiskers is used, the illuminant is self-heated up to a necessary radiation source temperature by means of a DC power source. In this case, a terminal portion of the illuminant on the positive electrode's side thereof, the temperature of which portion stays as low as around 500.degree. C., undergoes rapid progress of low-temperature oxidation despite the existence of the dense silica film on the surface thereof, whereby the serviceable life span of the radiation source is greatly affected. Thus, there is a need of taking a measure for prolonging the life span of an infrared radiation source comprising an illuminant.
The low-temperature oxidation behavior is also in relation with the relative density to theoretical of the material and the state of a silica film formed on the surface thereof. This behavior further differs from season to season. Since the rate of low-temperature oxidation is faster in summer than in winter, there has also been a suggestion that moisture in the air may be another factor in greatly affecting low-temperature oxidation.